Wnts are secreted proteins that regulate cell behavior through several pathways, the best characterized of which is Wnt/-catenin signaling, which plays many important roles in animal development and adult tissue homeostasis. For example, in intestinal crypts, the pathway is required to maintain stem cell populations, and is also needed to specify Paneth cells, which secrete anti-microbial proteins to keep gut bacteria in check. Misregulation of the pathway is associated with colorectal cancer (CRC) and inflammatory bowel disorders. Wnt signaling promotes nuclear accumulation of -catenin, which is then recruited to Wnt responsive cis- regulatory modules (W-CRMs) by members of the TCF/LEF1 (TCF) family of transcription factors (TFs). Once there, -catenin acts as a potent activator of Wnt target gene transcription. Several genome-wide studies have linked TCF occupancy with other TFs in the genome. This clustering of TFs has been termed a TF collective. However, the mechanisms underlying functional interactions between TCFs and other TFs are poorly understood. We have characterized two W-CRMs from the human axin2 and c-myc genes in detail using cell culture. The c-myc W-CRM is of interest because it contains a polymorphism in a TCF site linked to increased colorectal cancer (CRC) in humans. Our preliminary data support a model where TCFs work with several other TFs to achieve Wnt activation of these W-CRMs. We will characterize the physical and functional interactions between these factors, to understand how a TF collective containing TCFs operates. We have found that the c-myc W-CRM is active in several tissues in mouse embryos and given its link to CRC in humans, we will examine its activity in the adult mouse intestine. We will also test whether the factors important for activating this W-CRM in cell culture are also required in mouse tissues. One of these factors is Sox9, a TF which is which is typically thought of as an antagonist of Wnt/-catenin signaling, but which also cooperates with TCFs to activate the c-myc W-CRM. This cooperation may explain why TCFs and Sox9 are both required for Paneth cell formation in the intestine. In addition, Sox9 is required for testis formation and XY individuals with Sox9 mutations often develop as females. We are characterizating Sox9 mutants that specifically affect its ability to act with or against Wnt signaling. We propose to engineer mice with these mutations, to determine whether these activities underlie its role in Paneth cell specification and sex determination. This work will increase our understanding of the role of Wnt signaling in CRC as well as inflammatory bowel disorders that affect Paneth cell function.